1. Field of the Invention
The present invention relates to bit lines of a semiconductor device and a method for forming such bit lines, and more particularly, to bit lines of a semiconductor device having a Ti/CVD-TiN/W multilayer structure and a method for forming such bit lines.
2. Description of the Prior Art
Typically, bit lines of semiconductor devices are made of tungsten (W) exhibiting a high melting point and a low specific resistance. By virtue of the use of such a material, it is possible to achieve an increase in signal transmission rate and to use elongated bit lines. Accordingly, an increase in the design margin of semiconductor devices is obtained.
Conventionally, a double thin film consisting of a polysilicon film and a tungsten silicide (WSix) film has been used for bit lines of semiconductor devices in order to compensate for the high resistance exhibited in polysilicon. The polysilicon thin film exhibits a specific resistance of 1,000 .mu..OMEGA. cm or less at a thickness of 1,000 .ANG. whereas the tungsten silicide thin film exhibits a specific resistance of 700 .mu..OMEGA. cm or less. As a result, bit lines comprised of such a double thin film exhibit a more or less high resistance.
Where such bit lines are used for semiconductor devices which have an increased integration degree resulting in a reduction in line width, the resistance thereof increases, so that the signal transmission rate is limited. For this reason, it is necessary to use a metal exhibiting a very low specific resistance for bit lines in the fabrication of highly integrated semiconductor devices.
Such a problem can be solved by using a tungsten thin film which is formed in accordance with a chemical vapor deposition (CVD) method. This is because such a tungsten thin film exhibits a very low specific resistance ranging from 10 .mu..OMEGA. cm to 20 .mu..OMEGA. cm, and a superior step coverage capable of burying bit line contacts. However, such a CVD-W thin film results in an increase in contact resistance by 50% or more. The CVD-W thin film also causes damage to the semiconductor device when it is subjected to a thermal process such as a boro phosphor silicate glass (BPSG) reflow after its line formation. For this reason, a diffusion barrier should be used in order to suppress a reaction for forming tungsten silicide. A representative diffusion barrier material is titanium nitride (TiN). In terms of the forming method used for the diffusion barrier, such titanium nitride is classified into CVD-TiN and sputtered TiN. The CVD-TiN exhibits a superior diffusion barrier characteristic over the sputtered TiN. However, the CVD-TiN absorbs impurities, such as moisture and oxygen, existing in the atmosphere, thereby forming an oxide as an insulating layer on contacts in a subsequent process carried out at a high temperature.